1. Field of the Invention
This invention pertains in general to the field of breathing apparatuses and expiratory regulation thereof. More particularly the invention relates to regulation of expiratory pressure by means of an expiratory pressure regulator in such a breathing apparatus, and methods therefor. Even more particularly, the invention relates to regulation of expiratory pressure during an expiratory phase of a breathing cycle to obtain a desired positive end expiratory pressure (PEEP).
2. Description of the Prior Art
Various breathing apparatuses are arranged to provide a user-selectable desired positive end expiratory pressure (PEEP). Such apparatuses include intensive care ventilators, anesthesia machines, etc. However, the actual PEEP obtained may deviate from the user-selected desired PEEP for a variety of reasons. Such deviations may for instance occur due to changes in the patient situation or system during ventilation, e.g. a breathing cycle, such as patient changes, e.g. changes of patient position in a bed; system changes, like changes of tubing, filter changes, humidity moisture exchanger (HME) insertion, accumulation of humidity; etc.
In U.S. Pat. No. 6,564,798 of the same proprietor as the present application, a method for controlling an expiratory valve in a ventilator during expiration is disclosed. During expiration, the method comprises a first interval in which the expiratory valve is opened completely. During this first interval no active control of expiratory pressure is carried. A determination is made when the flow or pressure in the expiratory phase meets a predefined condition. A second interval is started when the flow in the expiratory part meets the condition. The expiratory valve is regulated during the second interval in order to attain a pre-set positive end expiratory pressure (PEEP) pressure in the patient. However, no details are given in U.S. Pat. No. 6,564,798 how this regulation is carried out. Furthermore, in breathing apparatus working on a bias flow, a new inspiratory cycle may be triggered when no gas flow is available for adjusting the expiratory pressure during regulation.
In US 2003/168066 A1 of the same applicant as the present application, an expiratory valve for regulating gas pressure within an expiration gas flow is disclosed. The regulation is carried out dependent on an input regulatory signal and an expiratory pressure sensor. A control unit is coupled to the expiratory valve and to the expiratory pressure sensor for calculating a target pressure as a function of time. The target pressure is dependent on a value of compliance calculated from measurements of pressure and inspired volume of provided breathing gas made during an inspiration phase. Compliance is calculated from ΔV, the change in volume, divided by ΔP, the change in pressure of the system comprising inspiratory tubing and the patient lung. Expiratory pressure is controlled based on the regulatory signal dependent on a magnitude of the difference between the target pressure and the actual pressure. However, reliability of the method disclosed in US 2003/168066 A1 may be improved, as compliance is a non-constant parameter and may even change during a single breathing cycle, i.e. the compliance calculated during inspiration may change during a single expiration. Compliance may e.g. change due to the patient moving, etc. In more detail, a major drawback with the method disclosed in US 2003/168066 A1 is that the pressure curve is defined during inspiration by means of a calculated time constant based on the calculated inspiratory compliance. However, the actual time constant in a real breathing circuit differs from this calculated time constant. The actual time constant is in fact dependent on the product of compliance AND the flow resistance. It is not possible to make any conclusion of the flow resistance and hence the time constant based solely on the value of the compliance. The flow resistance depends on the properties of the patient lung and the endotracheal tube, etc, and these factors are independent of the compliance. In addition, the actual time constant may change during the course of exhalation of a single breathing cycle, and thus the estimate disclosed in US 2003/168066 A1 is not suitable for a number of clinical situations.
Furthermore, it is desired that expiratory work of breathing of patients ventilated with breathing apparatuses is minimized.
Thus, there is a need for an improved method of regulating expiratory pressure and reliably achieving a desired PEEP.
Hence, an improved breathing apparatus and method of regulating expiratory pressure would be advantageous, and in particular allowing for increased flexibility, reliability, and patient safety would be advantageous.